Turkish artillery hit targets near Syria’s Tel Abyad border town for a second day on Thursday, killing several Syrian soldiers according to activists and security sources, after a mortar bomb fired from the area killed five Turkish civilians.

Turkey’s government said “aggressive action” against its territory by Syria’s military had become a serious threat to its national security and sought parliamentary approval for the deployment of Turkish troops beyond its borders.

“Turkey has no interest in a war with Syria. But Turkey is capable of protecting its borders and will retaliate when necessary,” Ibrahim Kalin, a senior adviser to Prime Minister Tayyip Erdogan, said on his Twitter account.

“Political, diplomatic initiatives will continue,” he said.

In the most serious cross-border escalation of the 18-month uprising in Syria, Turkey hit back after what it called “the last straw” when a mortar hit a residential neighborhood of the southern border town of Akcakale on Wednesday.

The Syrian Observatory for Human Rights said several Syrian soldiers were killed in the Turkish bombardment of a military post near the Syrian town of Tel Abyad, a few miles across the frontier from Akcakale. It did not say how many soldiers died.